Phytic acid is represented by the below Formula I.
Phytic acid or phytate is the hexa-phosphorus ester of inositol (1,2,3,4,5,6-cyclohexanehexolphosphoric acid), found in many seeds and cereals. It acts as the primary storage form of both phosphorus and inositol and accounts for as much as 50% of the total phosphorus content. Phytic acid in plants appears in the form of calcium, magnesium and potassium salts, which in general are called phytin. A large part of the phosphorus content of seeds is stored in these compounds. For example, about 70% of the total phosphorus in soybeans is accounted for by phytin. When the terms phytate or phytic acid are used herein, it is intended to include salts of phytic acid and molecular complexes of phytic acid with other soybean constituents.
All legumes contain phytic acid. However, soybeans have higher levels of phytic acid than any other legume. Phytic acid tends to form complexes with proteins and multivalent metal cations. Phytic acid complexes decrease the nutritional quality of soy protein. Phytic acid, because it interacts with multivalent metal cations, interferes with the assimilation by animals and humans of various metals such as calcium, iron and zinc. This may lead to deficiency disorders, especially for vegetarians, elderly people and infants.
Phytic acid also inhibits various enzymes in the gastrointestinal tract, including pepsin and trypsin and decreases the digestibility of soy protein. In addition, the phosphate present in phytic acid is not available to humans. Moreover, the presence of a relatively large amount of such unavailable phosphorus in infant food many lead to inadequate bone mineralization.
In typical commercial soy protein isolation processes, defatted soy flakes or soy flour are slurried with water and a base and extracted at pH values between 8.0 and 10.0 to solubilize proteins. The slurry is centrifuged to separate the insoluble part from the solution. The major fraction is recovered from the solution by precipitating at a pH near the isoelectric point of the protein (4.5), separating it by centrifugation, washing the precipitate with water redispersing it at pH 7 and spray-drying it to a powder. In such processes, phytic acid will follow the protein and tends to concentrate in the resulting soy protein product. The phytic acid content of commercial soy protein isolates is about 1.2–3%, whereas soybeans contain 1–2% phytic acid.
U.S. Pat. No. 2,732,395 (Bolley, et al., Jan. 24, 1956) discloses a method for separation of phytin from various oil seeds. The method involves acid extraction of an oil free seed meal or flour with aqueous acid at a pH within approximately the isoelectric range of the particular seed protein, generally about pH 4.5. The phytin is recovered from the soluble portion and the protein is recovered from the curd by extraction at a pH greater than 8 with separation of insoluble materials, and subsequent coagulation of the protein in the clarified alkaline extract by acidification, again within the isoelectric range of the protein. The method is applied to various oil seeds including defatted soybean flour to provide purified protein which is allegedly substantially free from organic phosphorous compounds.
U.S. Pat. No. 3,736,147 (Iacobucci et al., May 29, 1973) discloses an ultrafiltration process for the preparation of soy protein isolate having a reduced phytic acid content which involves various chemical treatments in combination with extensive ultrafiltration. Chemical treatment involves either enzymatic hydrolysis of the phytic acid by the enzyme phytase at neutral pH prior to ultrafiltration, ultrafiltration in the presence of calcium ion at low pH, or the use of ethylenediaminetetraacetic acid at a high pH.
U.S. Pat. No. 4,072,670 (Goodnight, Jr., et al., Feb. 7, 1978) discloses a basic flaw in prior art processes for the preparation of acid precipitated soy protein isolate as exemplified in the Bolley, et al., and Robbins, et al. patents cited above. The prior art precipitated the soy protein in the flake with acid in the presence of phytic acid. Goodnight, et al., found that an alkali stable complex is formed between the protein and the phytic acid under these circumstances which prevents dissociation of the phytin from the soybean protein at alkaline pH as is disclosed in the McKinney, et al. article cited above.
U.S. Pat. No. 4,697,004 (Puski et al., Sep. 29, 1987) relates to a high quality soy protein isolate with significantly reduced aluminum content and substantially free of phytic acid and phytate complexes that is prepared by aqueous extraction of defatted particulated soybeans at pH 8 to 10, and at a temperature above 65° C., separating the extract and then precipitating the protein out of solution, at a pH slightly higher than its isoelectric point, i.e., pH 5.3.
U.S. Pat. No. 5,248,765 (Mazer et al., Sep. 28, 1993) relates to a method for separating phytate and manganese from protein and dietary fiber that involves treatment of an aqueous slurry of phytate-containing material at a low pH with insoluble alumina.
European Patent 1,364,585 A1 (Fuji Oil Company, Ltd.) relates to producing a soybean protein which can be widely utilized in an acidic food of pH lower than 4.6 and is soluble in a range of pH 3.0 to 4.5, and whose solution has preferred transparency in appearance and excellent storage stability together with functional properties such as emulsifying and gel-forming capabilities. The reference shows that an original cloudy protein solution can be converted into a solubilized state having transparency by subjecting to the following treatments. The treatments subject a solution containing a soybean protein to either or both of (A) a treatment for eliminating or inactivating polyanionic substances which are derived from the protein source and contained in the solution, and (B) a treatment for adding a polycationic substance to the solution, as a treatment for increasing the positive surface charge of soybean protein in the system; and then subjecting the protein solution to a heat treatment at a temperature of above 100° C. in an acidic region of pH below the isoelectric point of the protein.